The present invention relates to generally to amplifier circuits, and more particularly, to a BiCMOS transconductor amplifier.
Received signals in wireless communications often vary in strength, as a result, variable gain amplifiers (VGA""s) are commonly used to allow a downstream signal processor to properly demodulate the wanted information. Desired characteristics for a VGA in a modern radio receiver include an exponential gain to linear control voltage characteristic, temperature stable gain, increased input linearity as the gain is reduced (increasing incoming signal strength), and a simple, low-current consuming circuit topology. While CMOS VGA""s are low cost and can be made highly linear, a straightforward, low-power means for generating the desired exponential (dB-linear) gain characteristic, with small process and temperature variation is not available.
Accordingly, a need remains for a simple, continuous VGA that exploits the benefits of NMOS devices, while still providing all the desired VGA characteristics mentioned above.
The present invention includes a variable-gain BiCMOS transconductance amplifier (VGA). In one embodiment, an NMOS differential pair amplifier with bipolar cascoding provides continuous gain control by adjustment of drain-source voltage to shift an input NMOS differential pair from a saturation region operation and high gain to a triode operation and low gain. A simple control circuit is used in order to generate the desired exponential gain to linear control voltage characteristic that is stable over temperature and process. The shift from saturation to triode operation of the input NMOS differential pair simultaneously increases the input linearity as the gain is reduced.
In one embodiment, the amplifier is based on an NMOS differential pair cascoded with a pair of npn bipolar devices. The gain is controlled by adjusting the drain-source voltage (VDS) of the NMOS differential pair via a control voltage (Vcontrol) applied at the bases of the npn devices. The resulting shift in VDS moves the NMOS devices from a saturation region in high gain to a triode region in low gain. The gain cell of the VGA incorporates a simple, robust structure with only four active transistors. The VGA employs a straightforward gain control circuit to achieve the desired dB-linear gain characteristic that is relatively insensitive to temperature and process variation. A further aspect of the VGA circuit is that the input linearity is enhanced as the gain is reduced.
In one embodiment, a variable-gain amplifier is provided that amplifies an input signal to produce an amplified output signal, and wherein the amplifier provides an exponential gain to linear control voltage characteristic. The amplifier comprises a control voltage generator that generates a first control signal and receives a second control signal. The amplifier also comprises an input transconductance stage that receives the input signal and provides the second control signal at an output terminal. The amplifier also comprises a cascode transistor pair that is coupled to the input transconductance stage, the cascode transistor pair receives the first control signal and generates the amplified output signal, and an output load coupled to the amplified output signal.
In one embodiment, a method is provided for amplifying an input signal to produce an amplified output signal, wherein the method provides an exponential gain to linear control voltage characteristic. The method comprises the steps of receiving the input signal at a transconductance stage that is coupled to a cascode transistor pair, generating a first control signal that is input the cascode transistor pair, receiving a second control signal that is provided at an output terminal of the transconductance stage that is also coupled to a current source, and providing a load that is coupled to the amplified output signal produced at an output of the cascode transistor pair.
In one embodiment, a radio receiver is provided that receives and processes radio frequency (RF) transmissions and includes a variable-gain amplifier that amplifies an input signal derived from the RF transmissions to produce an amplified output signal, and wherein the amplifier provides an exponential gain to linear control voltage characteristic. The amplifier comprises a control voltage generator that generates a first control signal and receives a second control signal. The amplifier also comprises an input transconductance stage that receives the input signal and provides the second control signal at an output terminal. The amplifier also comprises a cascode transistor pair that is coupled to the input transconductance stage, the cascode transistor pair receives the first control signal and generates the amplified output signal, and an output load coupled to the amplified output signal.